For some applications, it is desirable to be able to use the same aperture area for a plurality of array antennas. In such interleaved or common-aperture arrangements, the operation of each array antenna is complicated by the presence of the other array, which adds mutual coupling between elements and interelement spacing problems. This problem is particularly acute when elliptical or circular (or dual linear) polarization is desired. Such a situation is described in U.S. Pat. No. 5,258,771, issued Nov. 2, 1993 in the name of Praba, in which two separate arrays of helical antennas, operable at different or disparate frequencies, are interleaved on (or using) the same ground plane. As described by Praba, the mutual coupling problem is solved, at least in part, by making the antenna elements of opposite hands of polarization. Grating lobes attributable to spacing of antenna elements are reduced by designing the helices so that the radiation pattern of the individual elements superposes nulls over the grating-lobe peaks. It should be understood that the ground plane, as described by Praba, has dual uses, the first being as a physical support for the antennas. The second use of the ground plane is equally important, if less obvious, and that is the use as an electrical terminus or ground, which allows the individual antenna feeds to be accomplished by means of transmission-line structures. Those skilled in the art know that it is very important in many contexts, including the electrical feed of antennas, to form the structures as transmission lines rather than as simple electrical conductors. In this context, the term "transmission line" connotes various factors such as low standing-wave ratio (SWR), controlled impedances (either the same at each point along the transmission line, or with at least somewhat matched transitions between different impedance levels), and low leakage or losses, except at locations where signal power transfer is desired.
A multiband phased-array antenna consisting of an L-band microstrip patch array interleaved with a linearly-polarized X-band slot array is described in an article entitled A Multiband Phased Array Antenna, by Edward, B. J., et al., published in Proceedings of the Sixteenth Annual Antenna Applications Symposium, Sep. 23-25, 1992. Such antennas are more desirable than helical antennas for those situations in which a planar or two-dimensional structure is preferred to a three-dimensional structure such as that of the Praba arrangement.
Improved interleaved antenna arrays are desired.